Composite laminates, such as CFRP (Carbon Fiber Reinforced Plastic) material, i.e. a laminated structure composed of several layers of carbon fiber tape embedded in a synthetic resin, are of great interest for the aviation industry because of the high strength to weight ratio. For primary structures this high ratio is mainly achieved by optimizing the ply orientation and stacking sequence in a CFRP laminate, with respect to the direction of tensile and compressive strains acting upon the structure. A difference between the designed and produced stacking sequence can result in a drastic reduction of the mechanical strength of said structure. Other advanced composite laminates are for example aramide fiber and E glass fiber reinforced structures. Laminates of light weight metal plates, adhered together by means of a fiber embedded resin, are also of special interest for the aviation industry.
To check whether a laminated product meets the quality requirements forthcoming from airworthiness and safety regulations for aero space vehicles, several inspections have to be carried out. For example, during the last phase of manufacture there must be verified whether the prescribed manufacturing process has yielded a composite laminate in which the fiber/resin composition in all important areas meets the initial design phase. Further, upon repair it is also often necessary to determine the laminate composition in terms of stacking order and fiber orientation.
A non-destructive method for verifying the correct position of the load carrying fibers in a composite, is an ultrasonic method which is based on ultrasound refraction. The method is described in "Non-destructive Composite Laminate Characterization by Means of Ultrasonic Polar Scan", by W. H. M. van Dreumel and J. L. Speijer, in Materials Evaluation, vol. 39, no. 10 (1981), pp. 922-925. For the "Polar-Scan" method the laminate or product to be inspected is attached to a turn table and two ultrasonic tranducers are positioned at a certain angle at both sides of the laminate, one being used as a transmitter and the other as a receiver. The entire combination is placed in a tank which is filled with a liquid that permits transmission of longitudinal waves from the transmitter to the receiver. A transmitted signal is detected as an interference product of several wave modes and their reflections. This interference pattern strongly depends on the laminate configuration.
However, the "Polar Scan" method offers no direct quantitative data, in terms of stacking order and fiber orientation of a fiber reinforced laminate, but only a reasonable qualitative impression is obtained of the directional elastic properties of the laminate as a hole. A further drawback can be that the product under inspection has to be submerged in a tank. The method promises no small scale test instrument, for example to be used directly in the production line for carrying out an end-product inspection.
Yet another ultrasonic test method is given by EP-C-0,086,546. The advantage of the method described over the "Polar-Scan" method is that discrete data about the laminate stacking order and the layer orientation are produced. The described "CompoScan" device requires in principle no moving parts and the probe is simply placed on the surface of the laminate to be inspected. A contact fluid is necessary. However, the method does not offer reliable data in the case of laminates having a plurality of layers.
A more direct method of measuring the position of load carrying fibers in the composite laminate, used in practice, is the visual inspection by microscope of a transverse section of the laminate. However, the method gives information about the edges of the product only. When the number of weaves, their sequence and the orientation of the fiber varies from one location to another, due to the loading configuration of the structure, a full inspection of the end-product by microscope in a non-destructive manner is not possible. The only solution then is to take a sample of the laminate to be inspected, and making several angularly displaced transverse sections having different angles with respect to an edge or an imaginary edge of said sample. However, in practice such a destructive inspection is neither possible in most practical cases. Further, this direct method is not suitable for a quick inspection in, for example, a production line environment.
Another destructive, very time consuming and complex method for the inspection of composite laminates is the so-called pelling method, also used in practice. In this pelling method, the laminate to be inspected is heated above a temperature at which the synthetic resin, in case of a CFRP laminate the carbon fibers are embedded in, burns to leave the several layers of carbon fibers. By pelling these layers, the stacking order and fiber orientation can be easily determined. Although the results obtained with this method are very reliable, due to its complexity and its time consuming and destructive character, this pelling method is neither suitable for a quick inspection in, for example, a production line environment.